Factors for allowing an automobile to run safely can include a tire air pressure. An air pressure lower than an appropriate value may deteriorate a stable operation or fuel consumption and cause a tire burst. Thus, a tire air pressure alarm system for detecting a decreased tire air pressure to issue an alarm to a driver to prompt an appropriate procedure is an important technique from the viewpoints of environment protection and driver safety.
Conventional alarm system can be classified into a direct detection type and an indirect detection type. The direct detection type alarm apparatus includes a pressure sensor provided in a tire wheel to directly measure a tire air pressure. Although the direct detection type alarm apparatus can accurately detect a decreased air pressure, the apparatus has technical and cost disadvantages because the apparatus has a problem in the fault-tolerant performance in an actual environment and requires a high-cost sensing system for example.
On the other hand, the indirect detection is a method of estimating air pressures based on the rotation information of the respective wheels. The indirect detection can be further classified into the dynamic loaded radius method and the resonance frequency method. Among these methods, the dynamic loaded radius method applied in the present invention uses a phenomenon in which a tire having a decreased pressure is rotated at a higher speed than a tire having a normal pressure. The dynamic loaded radius method compares the rotation speeds of four tires of a vehicle to thereby detect a decreased pressure. Specifically, it is assumed that wheels at diagonal positions to each other are in a pair and a value of wheel speed comparison between two pairs is DEL1, a wheel speed comparison value between front wheels and rear wheels is DEL2, and a wheel speed comparison value between left wheels and right wheels is DEL3 (these values will be collectively referred to hereinafter as “DEL value”). When these comparison values reach a predetermined abnormal value, alarm is issued (see Patent Literature 1). This method can provide a simple calculation processing only based on wheel rotation speed signals obtained from wheel speed sensors. Thus, attention has been focused on this method as a low-cost detection system.